The present invention relates to an electronic device which can tune a voltage controlled oscillator (VCO) carrier frequency, pulse to pulse, to the carrier frequency of a radar pulse, operating over bands beyond one octave.
Such device can compensate for frequency errors up to .+-.500 MHz in a time period shorter than 100 .mu.sec. As the device is equipped with a memory circuit, signal lock may be maintained for the time sufficient to allow to any type of intentional interference.
The invention belongs to the radar field, and more exactly, to the field of "frequency tuning devices". It finds specific application in the field of Electronic Counter Measures. Here, the function of such devices is to effect in the shortest possible time, VCO tuning to the radar pulse carrier frequency. This way, further modulation of the VCO around such carrier, by means of noise or interference, may mask the radar return echo so as to deprive it of presence information in the case of surveillance and of tracking radars. At present, tuning problems are solved, in general, through devices such as:
PLL (Phase locked loop) PA1 AFC (Automatic Frequency Control) PA1 Intermediate frequency discriminator PA1 Interferometric frequency discriminator PA1 1.provide a discriminator/tuning circuit which can tune, pulse to pulse, the VCO frequency line to the radar pulse carrier frequency; PA1 2. correct errors greater than .+-.500 MHz within a time period not greater than 100 nsec; PA1 3. keep radar signal lock for the time sufficient to enable any type of intentional interference, such as &gt;100 .mu.sec, by means of a memory circuit inside the loop.
The PLL uses a phase detector as a comparator between input signal and VCO phase. This phase difference at the phase detector (PD) output is converted into a difference of potential, which applied to the VCO, changes its frequency in a way to reduce the initial phase error.
The AFC is based upon a frequency discriminator which may be of two types:
In the case of the IF discriminator, two resonant circuits tuned to frequencies symmetrical to a prefixed reference frequency are used; resonance curves, one of which inverted, (if the detected signal difference is calculated) give way to the typical S voltage-frequency curve of the discriminator, the null point of which coincides with the prefixed reference frequency.
In the case of the interferometric discriminator the input signal is split into two branches, one of which reaches a phase detector directly, while the other one reaches destination via a delay line.
The phase detector output is a voltage proportional to the phase between the two signals. This phase difference, in turn, is proportional to the input signal frequency.
The voltage proportional to the frequency difference, sent to the VCO, is such that through the AFC loop, the frequency received and that of the VCO are made to coincide.
In a tuning system using the PLL principle, there is a tight relation between lock-in band and loop band.
This means that when a lock-in band of a few hundred MHz is required, one PLL alone does not solve the problem adequately for obvious physical reasons relating to component bandwidth.
In a tuning system using the AFC principle, lock in time is related to the loop bandwidth, and the maximum .DELTA.f for which the loop can lock is exclusively set by the frequency discriminator characteristics. It is a well known fact that an IF type frequency discriminator has a limited lock in range (tens of MHz) while an interferometric discriminator, which gives way to wider lock in ranges, has the drawback of large deviations from linearity, as well as noise and dynamics problems. Furthermore lock-in times are longer and the loop is not suitable for ultrarapid tuning.
In any case these discriminators, working at fixed reference frequencies, do not give fast lock in because the frequency measurement, onto which lock in has to take place, and that of VCO, must be made on the same discriminating element at different times.